34675 datasheet(16/24 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34675

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

CHARGE CURRENT LIMITATION

The charge current is limited by multiple factors.

When the voltage difference between the input and the

battery (VIN-VBAT) is low, (VIN - VBAT) / RDS(ON), where

RDS(ON) is the on resistance of the power MOSFET, may be

less than the programmed CC mode current ICHG. In this

case, the charge current is limited by (VIN - VBAT) / RDS(ON).

When the voltage difference between the input and the

battery is too high, the large power dissipation may lead to

the charge current thermal foldback operation, due to the die

temperature regulation. The charge current is reduced to

prevent further temperature rise (See Charge Current

Thermal Foldback section for more information).

DC INPUT VOLTAGE

The 34675 accepts up to a 28V DC input. When all of the

following conditions are satisfied, the input is in a power-good

range for the charger to start charging. The conditions

include:

1. VIN > VPOR

2. VIN -VBAT > VOS

3. VIN < VOVP

where VOS is the offset voltage for the comparator that

monitors the input and the battery voltages. The VOS is for

preventing the reverse leakage current from the battery when

the external power supply is off. VOVP is the over-voltage

protection threshold. When the DC input voltage is above the

over-voltage protection threshold, the charger is disabled

internally. The 28V input voltage rating eliminates the need of

any additional input over-voltage protection circuitry.

CHARGE-ENABLE INPUT

The charge-enable input, EN, has a weak internal pull-

down current. Driving it to a low voltage, leaving it float, or

shorting it to ground, will enable the charger if the input

voltage is in the power-good range. Whenever the EN pin is

driven to a high voltage, the charger is disabled. When the

charger is disabled, the BAT pin does not output voltage and

current, the leakage current into the BAT pin is less than

1.0

μA, the output of the CHG pin is high, and the voltage at

the ISET pin is zero. If the input voltage is in the power-good

range when the charger is disabled, the output of the PPR is

still low to indicate the input voltage status and the DET pin

outputs voltage as well.

INPUT POWER PRESENCE INDICATOR

When VIN is applied and the voltage is above the power-

on-reset voltage threshold (VPOR), the PPR pin outputs a low

voltage to indicate the input-power presence. The PPR

output is only controlled by the input voltage. All other

functions, such as the EN pin, the over-voltage protection,

and the VIN-BAT comparator, do not affect the PPR output.

The PPR pin is capable of sinking at least 9.0mA of current

when outputting a low voltage to drive an external LED.

CHARGE STATUS INDICATORS

The 34675 has one charge-status indicator, CHG. CHG

outputs a low voltage when the charger is enabled and the

charging is in progress. When the charge cycle completes,

CHG outputs high-impedance. If the charger is disabled or

the input voltage is out of the power-good range, the CHG pin

outputs a high-impedance as well. The CHG pin has at least

9.0mA current-sinking capability to drive an external LED, the

same as the PPR pin.

CHARGE CURRENT THERMAL FOLDBACK

An internal thermal feedback loop begins to reduce the

charge current, when the die temperature reaches 120°C, to

prevent further temperature rise. This feature protects the

34675 from over-temperature failure, and allows the user to

push the limits of the power-handling capability of a given

circuit board, without the risk of damaging the 34675. The

charge current can be programmed according to the typical

(not the worst case) ambient temperature, with the assurance

that the charger will automatically reduce the current in worst

case conditions.

LINEAR REGULATOR

The linear regulator provides a 4.85V output with 10mA

current capability. The output is turned on when the input

voltage is above the POR threshold but lower than the OVP

threshold. The output is independent of any other signals in

the charger, such as the enable (EN) input.

When the input reaches the rising POR threshold, the DET

output starts to output a voltage, but is in the dropout mode.

The output voltage is determined by the dropout voltage. As

the input voltage rises above the regulation voltage, the

output is regulated at 4.85V. When the input voltage further

rises above the OVP rising threshold, the linear regulator is

turned off, and the output voltage drops to zero with an

internal 200k

Ω pull-down resistor. The output voltage

resumes to 4.85V when the input voltage falls below the OVP

falling threshold. The output voltage versus the input voltage

is shown in Figure 28, where the VPORR and VPORF are the

POR rising and falling threshold. The VOVPR and VOVPF are

the OVP rising and falling threshold. An output decoupling

capacitor of 0.1

μF to 1.0μF is required between the DET pin

and ground for the stability of the linear regulator.

BAT

ISET

1.0V

-------------- I

CHG

⋅

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